Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 AM EDT Sun Jul 30 2023 Valid 12Z Wed Aug 02 2023 - 12Z Sun Aug 06 2023 ...Hot temperatures and high heat indices from the south-central Plains and Mississippi Valley to the Southeast this week... ...Heavy Rain/flash flood threat to focus across the north-central Rockies states and vicinity midweek... ...Overview... An amplified and slow moving upper ridge will support above normal temperatures from the south-central Plains through the Mid-Mississippi Valley to the Southeast for much of this week along with high heat indices. Deepened monsoonal moisture lifting on the western periphery of the ridge this period will favor a slow shift of a main convective rainfall focus from the Great Basin to the north-central Rockies/High Plains. Meanwhile, shortwaves in northwest flow will spark strong to severe thunderstorms midweek across the Midwest to the Appalachians/Mid-Atlantic and up to the Northeast late week. This is also along and in advance of a organized surface low/frontal system that should moderate conditions in wake of passage. ...Guidance Evaluation/Predictability Assessment... Models and ensembles seem to still have a decent handle on the overall pattern which features various systems/shortwaves over our fine nation rounding the broad western and northern periphery of a re-positioned southern U.S. upper ridge. Some timing/details differences remain, most notably with the shortwave across and into the Great Lakes/vicinity later week. The 18/00 UTC GFS/GEFS runs seem slower outliers compared to the rest of the models and ensembles that offers better continuity. Accordingly, the WPC medium range product suite was mainly derived from a blend of the reasonably well clustered 12 UTC GFS/ECMWF/Canadian/UKMET and GEFS/ECMWF ensembles, along with the generally compatible 01 UTC National Blend of Models. Applied a bit greater blend weighting to the ECMWF/ECMWF ensemble mean to best maintain consistency with the previous WPC forecast. This also works well in general with the newer 00 UTC guidance cycle in a pattern with average to better predictability. ...Weather/Hazards Highlights... The greatest heat anomalies should settle this week across the south-central Plains to the lower Mississippi Valley, where above normal temperatures by 5 to 10+ degrees are likely as actual temperatures near or exceed 100F. The southern Plains to lower Mississippi Valley and Southeast may experience heat indices soaring above 105F, even above 110-115F in some areas. Record hot highs and warm minimum temperatures are widely possible in these regions next week. Areas from the Great Basin to the north-central Rockies/High Plains will see diurnally and terrain driven convection as fueled by deep monsoonal moisture and embedded shortwave energies on/around the periphery of the main upper ridge. QPF, moisture, and instability fields seem to support slight risks of excessive rainfall continuing into Wednesday and Thursday across portions of central Wyoming and north-central Colorado where flash flood guidance values are lower and burn scars. Meanwhile, the north-central U.S. can also expect scattered storms as shortwaves/upper troughing work into the region. A heavy rain/flash flooding threat will likely be tied to the potential for organized convective systems in the northwesterly flow. Marginal Risks have been included on both Days 4 and 5 to cover this potential from the Midwest to the Appalachians/Mid-Atlantic, with eventual more concentrated Slight Risk areas possible. This activity will likely work across the Northeast later week in a pattern to monitor. Activity will also be aided as an organized low pressure system subsequently tracks eastward across Canada, as a cold front trails back across the lower 48 and stalls back into the central Plains. This all occurs well to the north of a lead cold front pushed earlier period unseasonably far south to offer a more defined than normal summertime convective focus over Florida. There could be some lingering localized high rain rates given stalling moisture pooling/instability focus as the front slowly weakens over time. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw